The present invention relates to nuclear magnetic resonance (MR) imaging and more particularly to a novel method for performing Fourier velocity imaging in combination with MR angiography. The inventive method preferably makes use of a multiple overlapping thin slab (MOTSA) data acquisition method which gives a high-resolution, low-noise signal.
MR imaging techniques can be utilized to detect flowing fluids non-invasively, and are particularly valuable for selectively detecting and imaging the flow of blood in living organisms. Other techniques, such as standard X-ray dye angiographic techniques which require injection of a contrast dye via a catheter, may be used for imaging blood. However, such methods are invasive and not totally benign. MR imaging is non-invasive and is thus much more desirable as a technique for obtaining angiographic images.
Information about blood flow can be of critical importance in diagnostic use. For example, atheromatous plaque, which commonly forms at the bifurcations of the carotid arteries, can cause hemodynamically significant stenosis which may cause transient ischemic attack. Atheromatous plaque can be detected with blood flow imaging techniques. It is desirable to detect atherosclerotic plaque before the occurrence of ulcerization which may result in a brain embolism and stroke. The bifurcations of the carotid arteries are only one area of the human body in which blood flow information is of significance for diagnostic use. The methods described in this specification can be applied equally well to other areas and to subjects other than the human body.
In MR imaging, the object to be imaged is placed in a static magnetic field which causes the spins of the atomic nuclei within the object to become aligned with the static magnetic field. A radio frequency (RF) magnetic pulse applied in a direction perpendicular to the static field causes a change in the state of the nuclei. Following the RF pulse, the nuclei relax back to their original state; the energy released during this relaxation process constitutes the MR signal or "echo." Gradient magnetic fields superimposed on the static magnetic field are used to select the area within the object which will be imaged during a given data acquisition cycle.
MR techniques can be classified as either two-dimensional (2D) projection (similar to conventional X-ray angiography) or three-dimensional (3D). 2D projection imaging techniques, which directly acquire a projection through the subject, are relatively fast but generate only a single view of the areas of interest. Further acquisitions are necessary to obtain additional views. More significantly, all projection techniques are very sensitive to phase dispersion along the projection direction and there is generally significant signal loss due to this phase dispersion.
It is possible to combine many thin slice (non-projection) 2D images to make a multiple thin slice image of a three-dimensional sample volume. Multiple thin slice techniques of this type obtain reasonable images of large and small vessels. However, the images are very noisy because of the small number of signal measurements which are used to generate the image compared to the number of signal measurements used in 3D imaging. There is also signal loss from velocity dependent phase dispersion due to the moderately large slice thickness, which is typically not less than 2 mm.
3D imaging techniques obtain measurements of object density at small points distributed throughout the three dimensional region to be imaged. In 3D imaging techniques, MR signals are measured simultaneously from a three-dimensional volume rather than from individual slices one after another as in two-dimensional imaging. A 3D Fourier transformation is then performed to determine the value of each voxel (the volume unit for which a single measure of the signal is obtained for display as part of the image). The benefits of using a 3D imaging technique include lower noise and smaller voxel dimensions. 3D acquisition techniques have the disadvantage of low blood signal due to the thickness of the 3D slab imaged. Because of the slab thickness, blood remains in the slab for a significant fraction of the imaging time and is saturated by the RF pulse. The saturation of the blood causes its signal to be weaker than the signal from unsaturated, inflowing blood. Such a decrease in signal is especially significant in small vessels with slow blood flow. The loss in vessel detail due to the time blood spends in the slab, as well as to phase dispersion across the dimensions of the voxel, can be reduced with the use of thinner slabs and thus fewer and/or smaller voxels in the acquisition. However, this decreased slab thickness results in a thinner field of view which limits the diagnostic utility of the technique.
The quality of MR spatial images can be improved by the use of a multiple overlapping thin slab acquisition (MOTSA) technique which combines the advantages of 3D MR techniques (lower noise and smaller voxel dimensions) with those of 2-dimensional MR techniques (thin excitation volume which results in minimal signal loss due to RF saturation) and eliminates the drawbacks of both techniques.
MR techniques can be used to measure blood flow velocities, as well as to provide spatial information about flowing blood. By use of a bipolar magnetic gradient pulse in a particular direction, an MR signal can be produced which encodes only the velocity components of blood flow in that direction. The resolution of the velocity measurements can be improved by separating in time the lobes of the bipolar gradient pulse. In addition, the contrast between the flowing blood and stationary tissue can be improved by using a fast gradient echo technique and adjusting the RF gradient spoiling pulses to suppress the signal from stationary tissue.
Prior to the development of MOTSA, it was not possible to obtain data which could be used to simultaneously generate both spatial and velocity images of sufficiently high quality to be useful with the 2D and 3D data acquisition methods available. Velocity measurements employing MOTSA are taken from a small enough region and of sufficient resolution to be usable. Prior art methods may have adequate resolution of the velocity but the measurements are not taken for a small enough region to be useful. None of the prior art methods allow for the simultaneous acquisition of anatomical and velocity images. The slice thicknesses used in 2D and 3D imaging make velocity measurements made with these techniques highly sensitive to pulsatile blood flow. Pulsatile flow, as found in arteries, causes phase dispersion which results in signal suppression.
The inventive method presented herein provides for the simultaneous measurement of spatial and velocity signals, with high resolution and signal-to-noise ratio.